(1) Field of the Invention
The present invention relates to a method for producing optical glass by sintering a gel having a high silica content, which is obtained by hydrolyzing a metal alkoxide. More particularly, the present invention relates to a method for producing optical glass by forming a high-silica-content gel having a predetermined shape free of cracks and sintering this gel.
(2) Description of the Prior Art
As the method for producing glass by utilizing the hydrolysis reaction, there is known a method comprising hydrolyzing a silicon alkoxide or a mixture formed by adding an alkoxide of a desired additive element to a silicon alkoxide, to form a silica gel or additive element oxide-containing silica gel, and heating and sintering the gel at a temperature higher than about 1000.degree. C. but lower than the melting point of the gel. This method is disclosed in, for example, M. Yamane et al., Journal of Materials Science, vol. 14 (1979), pages 607-611.
Recently, optical fibers have been used as various optical communication media. These optical fibers are prepared from high-silica-content glass (containing a dopant for adjustment of the refractive index according to need). As the method for preparing these optical fibers, there are known (i) a method in which the starting material is molten in a crucible and the melt is formed into fibers, (ii) a method in which a film of high-silica-content glass is formed on the inner wall of a quartz tube by the CVD method (chemical vapor deposition method) and the film is collapsed and drawn at a high temperature to obtain fibers, and (iii) a method in which glass soot is formed by the CVD method, the soot is piled up, the piled soot is sintered to form a mother rod composed of high-silica-content glass and the mother rod is drawn at a high temperature to obtain fibers.
However, these conventional methods are still insufficient and defective. For example, in the method (i), it is difficult to obtain glass having a high silica content, and since purification of the starting material is difficult and incorporation of contaminants from the crucible cannot be avoided, it is difficult to obtain optical fibers having a high purity. In the method (ii), high-efficiency mass production is impossible and multi-component glass (containing a considerable amount of Na) cannot be prepared. Moreover, since the thermal oxidation reaction is utilized, when P, Ge or B is used as the dopant element, the yield is low, and production equipment becomes expensive. The method (iii) is defective in the multi-component glass cannot be prepared, high-efficiency mass production is impossible and production equipment becomes expensive. Furthermore, according to any of the methods (i), (ii) and (iii), a glass body having a desired shape cannot freely be prepared. Moreover, since silicon oxide is formed through a high temperature treatment in the methods (ii) and (iii) or since glass should be sufficiently molten at a high temperature in the method (i), increase of the manufacturing cost cannot be avoided.
As a means for eliminating these disadvantages of the conventional methods for preparing optical fibers, a method has been proposed for preparing a mother rod for production of optical fibers, which comprises hydrolyzing a metal alkoxide (where Si and Ge are contained as the metal) and sintering the resulting high-silica-content gel. A patent application was filed for this method in Japan as Japanese Patent Application No. 3957/79, and corresponding applications were filed in U.S.A., Great Britain, West Germany, France, Netherland and Canada claiming the Convention priority based on the above Japanese application. All of these prior applications are still pending, and the above-mentioned method is not prior art to the present invention.
When a mother rod is drawn at a high temperature to form fibers as pointed out above, the mother rod should naturally have a predetermined shape. Since the mother rod obtained by sintering a high-silica-content gel is required to have a predetermined shape, it is required that the high-silica-content gel before sintering (dry gel described hereinafter) should have a predetermined shape free of defects such as cracks. Also in the case where the mother rod is applied to optical uses other than production of optical fibers, the mother rod is similarly required to have a predetermined shape and be free of defects such as cracks. According to the conventional methods, however, it is considerably difficult to produce stable high-silica-content gels (dry gels) having a predetermined shape free of cracks.
In the method for producing glass by hydrolyzing a silicon alkoxide such as Si(OCH.sub.3).sub.4 or Si(OC.sub.2 H.sub.5).sub.4 as the main starting material, optionally with an appropriate dopant, to form a high-silica-content gel, and treating the gel at an appropriate high temperature in an appropriate atmosphere to effect sintering and conversion of the gel to glass, it is one of the important requirements that a faultless high-silica-content gel having a desired shape should be stably obtained as the material to be sintered.
In the specification of the above-mentioned pending application (which is not prior art), it is taught that a mixed solution ordinarily obtained by adding water and alcohol to Si(OCH.sub.3).sub.4 or the like (the mixed solution is converted to a sol with the lapse of time, but in the instant specification, also such sol is included in the mixed solution) is charged into a vessel and is hydrolyzed at an appropriate temperature in the range of from 10.degree. to 120.degree. C. to cause gelation, and the obtained gel is gradually dried at a temperature of 50.degree. to 120.degree. C. to remove a solution of CH.sub.3 OH or the like formed as the reaction product at the hydrolysis and excessive H.sub.2 O and obtain a dry gel. At this step, after the solution has fallen into the passive state, that is, the gell state, CH.sub.3 OH and H.sub.2 O contained in the interior of the gel is allowed to ooze out by a dehydrating condensation reaction or the like, and contraction of the gel is initiated and the gel separates from the wall of the vessel. Namely, the phenomenon called "syneresis" in the colloidal chemistry takes place. Cracks are readily formed at the time of this syneresis, rendering it difficult to obtain a dry gel having a desired shape.
In order to form a dry gel free of cracks, it is necessary to dry a gel at a predetermined drying speed at the above-mentioned drying step. According to the teaching of the specification of the above-mentioned pending application (which is not prior art), such drying speed can be attained if the weight reduction rate at the drying speed is less than 40%/day, preferably 10 to 20%/day, based on the weight of the gel in the initial stage of gelation. At the step of drying the gel, cracks (other than cracks formed at the time of syneresis) are formed if particles as structural units of the gel are not allowed to effect re-arrangement throughout the gel when solutions of H.sub.2 O and CH.sub.3 OH or the like in the interior of the gel are being removed. Accordingly, if the gel is gradually dried over a period sufficient to cause re-arrangement of the particles throughout the gel, a crack-free dry gel having a like figure to the vessel can be obtained. It is not easy to make such adjustment of the drying speed only by perception. Therefore, it often happens that at the step of drying the gel, the drying speed is partially deviated from the appropriate range and cracks are formed in the gel. Furthermore, even if cracks are not formed, this drying step is not preferred from the industrial viewpoint because an step is complicated and troublesome.
The following references are cited to show the state of the art:
(i) Japanese Patent Publication No. 6604/73 PA1 (ii) Japanese Patent Application Laid-Open Specification No. 137086/78 PA1 (iii) M. Yamane et al., Journal of Materials Science, vol. 13 (1978), pages 607-611